On February 9, 2007, the aerosol community lost one of its most respected members. Sheldon K. Friedlander died at age 79 of complications from pulmonary fibrosis at his home in Pacific Palisades. He was the Parsons Professor of Chemical Engineering at University of California, Los Angeles (UCLA). Sheldon's remarkable career, which spanned six decades, began with a B. S. from Columbia University and a Masters degree from MIT, both in Chemical Engineering. He also worked at the Harvard School of Public Health on an Atomic Energy Commission project regarding control of radioactive aerosols. His time at Harvard catalyzed an interest in aerosols, which he pursued through subsequent Ph.D. research in Chemical Engineering at the University of Illinois at Urbana-Champaign. He then served as a faculty member at Columbia University (1954-1957), John Hopkins University (1957-1964), California Institute of Technology (1964-1978), and UCLA (1978-2007). Sheldon started his Ph.D. studies at a time when the field of aerosol science was in its early stages of development. Working with H. F. Johnstone, he focused on how particles in turbulent airflow are deposited on the walls of pipes and ducts. Sheldon made important contributions right from the start: he introduced the notion of a "stopping distance" of a particle injected into stagnant air, and then used this concept to predict particle motion through the viscous boundary layer to the surface. His thesis work laid the foundation for much of the later work on deposition of particles in industrial systems as well as dry deposition from the ambient atmosphere, where turbulent eddies impart velocities normal to the mean flow and enable particles to reach the surface.